1. Field of the Invention
The present invention is directed to a method for the production of valuable hydrocarbon products by reacting a carbonaceous material and steam in a molten metal to form a synthesis gas that can be used to produce high-value hydrocarbon products. More particularly, the present invention is directed to a method for the production of a synthesis gas that includes a controlled ratio of hydrogen to carbon monoxide by contacting a carbonaceous material and a reactive metal with steam, wherein a portion of the steam reacts with the carbonaceous material and a portion of the steam reacts with the reactive metal. The synthesis gas can be used to form high-value hydrocarbon products, such as methane or methanol.
2. Description of Related Art
Recently, the United States and other countries have experienced a shortage of natural gas and as a result, natural gas prices for consumers have increased substantially. Accordingly, there is a pressing need for economic methods for the manufacture of a high-value heating gas that can be used in place of natural gas. Natural gas has a composition that includes from about 80 percent to 93 percent methane (CH4), the balance including ethane (C2H6), propane (C3H8), butane (C4H10) and nitrogen (N2). Methane, the primary component of natural gas, has a heating value of about 21,520 Btu/lb. Thus, an economic method for the production of methane would supplement the use of non-renewable natural gas.
There are many natural resources in addition to natural gas that are utilized to produce energy. For example, coal can be burned in conventional boilers to generate steam, which is converted to energy through steam turbines. 85 percent of the electricity in the United States is generated by combusting fossil fuels, namely coal, oil and natural gas. Coal however, because of its high carbon content, generates large quantities of carbon dioxide (CO2), and the use of coal for electricity generation is a major contributor to the 5.5 billion tons of CO2 emitted by the United States per annum. The 5.5 billon tons of CO2 amounts to one-fourth of the world emissions. Coal combustion is also responsible for other pollution, most notably sulfur dioxide (SO2) and nitrogen oxides (NOx), both of which are now regulated.
Furthermore, only 30 percent of the heat generated by burning coal is converted into electricity and 70 percent is wasted to the atmosphere. In contrast, electrical generation in modern plants burning natural gas is about 50 percent efficient and natural gas produces only about 60 percent of the CO2 that coal produces.
As an alternative to simply burning high carbon containing materials, such as coal, the materials can be converted to a synthesis gas in a gasifier. Synthesis gas includes five major gaseous components—carbon monoxide (CO), hydrogen (H2), methane, carbon dioxide and steam (H2O). These gases are derived from the carbon (C), hydrogen, and oxygen (O2) molecules found in the high carbon containing material and steam used to convert the high carbon containing material to synthesis gas. Other elements, designated impurities, typically found with carbonaceous materials include sulfur (S), nitrogen (N2), chlorine (Cl2) and fluorine (F). These impurities can form minor amounts of other gaseous species. Taken together the major and minor gases constitute a “raw” synthesis gas stream. As used herein, synthesis gas refers to the gas mixture after the minor gases have been removed. Nitrogen, steam and carbon dioxide do not contribute to the heating value and therefore typically are reduced or eliminated from the gas stream. The term “syngas” refers to a gaseous mixture that includes only hydrogen and carbon monoxide.
Synthesis gas has numerous applications, including the conversion of the synthesis gas into valuable hydrocarbons. In one application, the synthesis gas can be converted to methane, which is burned in a combined cycle power plant to generate electricity. The combined cycle gas turbines can be located at coal-fired generating stations thereby taking advantage of existing coal-handling infrastructure and electrical transmission lines. Most importantly, compared to coal-fired electrical generators, the conversion efficiency of thermal to electrical energy increases by about 67 percent. Concomitantly, there is a reduction in carbon dioxide emissions per unit of electricity.
For a gas turbine, gas is input to the turbine and the output is thermal energy. For increased efficiency, a gas with a high thermal energy per cubic foot is desirable. The net heating value (heat of combustion) of the three major components of synthesis gas are illustrated in Table 1 below. These values assume that the heat contained in the steam, the combustion product of hydrogen, is not recovered.
TABLE 1Net Heats of CombustionSynthesis GasComponentBtu/lbsBtu/ft3Carbon Monoxide4,347322Hydrogen51,623275Methane21,520913
As is illustrated in Table 1, methane releases more than three times the amount of heat that hydrogen releases on a per cubic foot basis. The reason for this is that hydrogen occupies more cubic feet on a per pound basis, even though hydrogen has more Btu on a per pound basis. Due to its clean burning nature and high heat content, methane is the preferred fuel. Consequently, syngas (H2 and CO) is more economically burned after it is converted to methane.
Syngas can be used to form other hydrocarbons in addition to methane. Since 1955, SASOL, a South African entity has been producing a waxy synthetic crude from syngas. Some transportation fuel, about 11 percent gasoline, is extracted from the synthetic crude. However, due to the large portion of hydrocarbons having a high molecular weight and oxygenated organics that are also produced, other approaches have been investigated for making specific materials from syngas.
There are well known processes for producing methanol (CH3OH) and acetic acid (CH3COOH) from syngas, for example. Typically, methanol is produced using syngas derived from natural gas, which exerts further pressure on the price and availability of natural gas. At least one major US oil company has developed a family of catalysts that produce a mixture of hydrocarbons in the gasoline range with high selectivity from methanol. Because methanol can be readily made from syngas, and catalysts are available for converting methanol into gasoline with great selectivity, coal-derived syngas affords the US an opportunity to achieve energy independence.
Methanol is also a chemical building block for manufacturing a wide array of other products, including: MTBE (methyl tertiary butyl ether) used in reformulated gasoline; formaldehyde resins, used in engineered wood products and products such as seat cushions and spandex fibers; acetic acid used to make PET (polyethylene terepthalate) plastic bottles and polyester fibers; and windshield wiper fluid. Additionally, methanol is relatively environmentally benign, is less volatile than gasoline and is a leading candidate to power fuel cell vehicles.
There are known processes for converting coal into gaseous products. Hydrogasification converts coal and steam into a raw synthesis gas. Gasification, a companion process, employs coal, steam and oxygen and produces hydrogen, carbon monoxide and carbon dioxide, but no methane. Pyrolysis, which utilizes heat alone, partitions coal into volatile matter and a coke or char. The volatile matter includes hydrogen, oxygen, some portion of the carbon (volatile carbon), organic sulfur and trace elements. The coke or char includes the balance of the (fixed) carbon and the ash derived from the mineral matter accompanying the organics.
Heat by itself disproportionates gaseous volatile matter, derived from coal, into methane and carbon as is illustrated by Equation 1.CHx→(x/4) CH4+[1−(x/4)]C  (1)                (where the value of x must be less than 4)        
The hydropyrolysis reaction combines hydrogen and volatile matter to form methane and carbon. This reaction, illustrated by Equation 2, is exothermic.CHx+m H2→[(x+2m)/4]CH4+{1−[(x+2m)/4]}C  (2)                (where the sum of x plus 2m must be less than 4)        
The following solid-gas chemical reactions are applicable to the hydrogasification of organics at temperatures above 1200° C. The highly exothermic reaction of carbon and oxygen illustrated by Equation 3 can be a primary source of process heat, producing about −394 MJ/kg-mole of heat.2 C+O2→2 CO  (3)
The hydrogasification of carbon is also an exothermic reaction, illustrated by Equation 4, yielding −75 MJ/kg-mole of heat.C+2 H2→CH4  (4)
The steam-carbon reaction illustrated by Equation 5, is highly endothermic, requiring +175 MJ/kg-mole of heat.C+H2O→CO+H2  (5)
Gas phase reactions, applicable to the formed fuel gases at temperatures below 1000° C. include the mildly exothermic (2.8 MJ/kg-mole) water gas shift reaction, illustrated in Equation 6.CO+H2O→H2+CO2  (6)
The highly exothermic methanation reaction is illustrated in Equation 7 (−250 MJ/kg-mole).CO+3 H2→CH4+H2O  (7)
Gasification is the process step that converts a solid (or liquid) fuel into a gaseous fuel by breaking (disassembling) the fuel into its constituent parts (molecules). When gasified with steam and oxygen, organic material is converted into a synthesis gas that may include five gaseous components: carbon monoxide, hydrogen, methane, carbon dioxide and steam.
The concentration of the individual product gases (all reactions above) all move in the direction of thermodynamic equilibrium, limited by kinetics, which is strongly related to temperature. The temperature of the gasifier, therefore, is the predominate factor that determines which gaseous species will form and in what amount. FIG. 1 illustrates the influence of temperature on a mixture of gases (four parts hydrogen and one part each carbon monoxide and methane) allowed to come to thermodynamic equilibrium. This mixture was thermodynamically equilibrated at various temperatures, over a range from 200° C. to 1200° C. In addition to temperature, the type of gasifying equipment (moving bed, fluidized bed or entrained flow) also exerts a strong influence on the resulting synthesis gas mix.
Equation 8 illustrates the ideal coal hydrogasification reaction.Coal+H2O →CH4+CO2  (8)
The ideal hydrogasification reaction illustrated by Equation 8 is slightly endothermic and is favorable for methane production only at low temperatures, where the kinetics are too slow to be commercially useful. To circumvent this thermodynamic dilemma, various hydrogasification processes have been proposed for coal. These processes conduct a sequence of related chemical reactions such that the sum of the reactions is identical to the ideal reaction of Equation 8. One sequence of reactions includes gasification to convert the solid fuel, coal or other organic material, into a gaseous fuel by reacting it with steam and usually oxygen at high temperatures and in an entrained flow gasifier. This gasification step produces a gas comprised predominantly of hydrogen and carbon monoxide, with some impurities. Typically, the resulting ratio of hydrogen to carbon monoxide (H2:CO) for entrained flow reactors falls between 0.5 and 0.8. The water gas shift reaction can be used to increase the ratio of hydrogen to carbon monoxide by subtracting carbon monoxide from the system. This is done by reacting carbon monoxide with additional steam to produce carbon dioxide. Sulfur and other impurities can also be removed from the raw synthesis gas. The resulting carbon dioxide can be removed by pressure swing adsorption or amine scrubbing. Finally, the scrubbed syngas, with the proper H2:CO ratio, is passed over appropriate catalysts to produce, for example, methane (3:1 ratio) or methanol (2:1 ratio).
The H2:CO ratio produced by other gasifying systems varies, as shown below in Table 2. (Data taken from Perry's Chemical Engineers' Handbook, 7th ed, 1997, Table 27-11)
TABLE 2Gasifying SystemsCOMMERCIALGASIFYING SYSTEMH2:CO RATIOMoving Bed TypeLurgi1.77BG Lurgi0.58Fluid Bed TypeKRW (Air)0.63KRW (Oxygen)0.51Entrained FlowShell0.42Texaco0.77
The H2:CO ratio for the above gasifiers is less than 1.9. The Lurgi gasifying process has the highest H2 to CO ratio, however, it can only utilize coal in the size range of 2 mm to 50 mm. The resulting requirement for disposal of material smaller than 2 mm imposes an onerous economic burden on the Lurgi process. An example of a gasifier having a rotatable grate is disclosed in U.S. Pat. No. 3,930,811, by Hiller et al. The H2:CO ratios for all other gasifiers listed in Table 2 is less than 1. These ratios are established primarily by the reactor type and the type of coal. This fixed ratio, unique to each gasifier-coal combination, occurs because all of the gasifiers above use oxygen to supply adequate input heat to secure a process heat balance for a specified coal and steam rate. Any change in coal rate or steam rate, for the intended purpose of affecting the H2:CO ratio, would destroy the heat balance. Prabhakar G. Bhandarkar in an article entitled Gasification Overview Focus on India, Hydrocarbon Asia, November/December 2001, discusses various gasification systems including some of those listed in Table 2.
Molten metal gasification is one technique for gasifying coal. An example of a molten metal gasification process is disclosed in U.S. Pat. No. 4,389,246 by Okamura et al. issued Jun. 21, 1983 and assigned to Sumitomo Metal Industries. Okamura et al. discloses an example (Example 1) wherein coal and steam was fed into a furnace containing molten iron at 1500° C. The coal was fed at a rate of 3.5 tons per hour and the steam was fed at a rate of 400 kg/hr (0.44 tons/hr). The steam and coal were blown onto the surface of the molten metal along with oxygen at high velocities to produce a depression of a specified geometry. The average gas production was 7500 Nm3/hr. The actual composition of the gas as reported by Okamura et al. was:
Carbon Monoxide62.5%Hydrogen33.9%Oxygen0.02%Nitrogen 1.4%Carbon Dioxide 2.0%Total Sulfur<80 ppm
The Okamura et al. example is typical of oxygen-blown slagging gasifiers. The critical parameter from the example is the H2:CO ratio, which is only 0.54:1 (33.9/62.5). In contrast, a 3:1 or 2:1 ratio is necessary to produce methane or methanol, respectively.
In addition to the Okamura et. al. patent there are other known processes for producing synthesis gas from steam and carbon. For example, U.S. Pat. No. 1,592,861 by Leonarz discloses a method for the production of water gas (primarily H2 and CO) by contacting steam with uncombined carbon in a bath of molten metal. The steam is dissociated into its constituent elements by carburetion at temperatures of 900° C. to 1200° C. The carbon combined with the oxygen of the gas is sufficient in quantity to produce carbon monoxide but not to make an appreciable quantity of carbon dioxide.
U.S. Pat. No. 2,953,445 by Rummel discloses the gasification of fuels and decomposition of gases in a molten slag bath. It is disclosed that a water gas composition is obtained composed primarily of hydrogen and carbon monoxide wherein the ratio of hydrogen to carbon monoxide is about 0.38:1.
U.S. Pat. No. 4,187,672 by Rasor discloses an apparatus for converting carbonaceous material into fuel gases. For example, raw coal can be gasified in a molten metal bath such as molten iron at temperatures of 1200° C. to 1700° C. Steam is injected to react with the carbon endothermically and moderate the reaction.
U.S. Pat. No. 4,388,084 by Okane et al. discloses a process for the gasification of coal by injecting coal, oxygen and steam onto molten iron at a temperature of about 1500° C. A gas product is produced having a ratio of hydrogen to carbon monoxide of about 0.5:1.
U.S. Pat. No. 5,645,615 by Malone et al. discloses a method for decomposing carbon and hydrogen containing feeds, such as coal, by injecting the feed into a molten metal using a submerged lance.
Donald B. Anthony, in a 1974 Thesis entitled “Rapid Devolatilization and Hydrogasification of Pulverized Coal,” found that rapid heating of coal in the presence of hydrogen can increase the amount of volatile matter significantly. Under thermal decomposition, different chemical bonds rupture at different temperatures. The rupturing bonds release volatiles and initiate char-forming reactions. Short-lived (<1 second) intermediaries in the char-forming sequence can react with hydrogen to form additional volatile matter. It was also found that freshly devolatilized coal is more reactive than pretreated coal. Further, the carbon that is residual from freshly devolatilized coal may possess excess free energies. The equilibrium constant for the hydrogasification reaction may be larger by a factor of 10 or more.
A significant limitation of the foregoing methods for producing syngas is that the synthesis gas must be treated to remove carbon oxides before the gas product can be used to produce high-value products such as methane or methanol. Process steps to eliminate carbon oxides from the gas stream are relatively costly. It would be advantageous to provide a method that can provide a synthesis gas having a controlled ratio of hydrogen to carbon monoxide, and in particular where the molar ratio of hydrogen to carbon monoxide is at least about 1:1, such as at least about 2:1, for the subsequent formation of high-value hydrocarbons.